Hmm, so what attitude does she have? Have you noticed her eating at night? This is the way to increase her capacity for sure, you get her maximum stuffed and then go to sleep
Her attitude is i believe around or just under 100, I have her full capacity almost the entire day either through cooking or ice cream every day. I've only managed to get her over capacity 2 or 3 times but it immediately reset after I checked her stats again